1. Field of the Invention
The present invention relates to superconducting cables, and particularly to a multilayer high-temperature superconducting cable in which alternating current (AC) loss can be reduced.
2. Description of the Related Art
Advances have been made in the development of compact, bulk-power-transmission superconducting cables. Superconducting cables generally have a multilayer cable structure in which a plurality of layers of superconducting tapes are spirally wound around a core material (former). Superconducting tapes are Bi-based Ag-sheathed superconducting tapes having Bi-based high-temperature superconducting filaments embedded in an Ag matrix, or the like.
In such a multilayer cable structure, for example, when the tapes are wound with a constant pitch in winding directions which are alternately opposite with respect to adjacent layers, a nonuniform current flow occurs due to the difference in impedance among the layers. That is, a larger current flows in an outer layer of lower impedance, and a smaller current flows in an inner layer of higher impedance. Such nonuniform current flow causes each layer to behave electromagnetically as a single block (i.e., so-called mono-block model), which results in high AC loss.
Accordingly, the phenomenon of nonuniform current flow must be suppressed in order to reduce the AC loss. One of the countermeasures for suppressing nonuniform current flow is to achieve uniformity of impedance across the layers. Specifically, the winding pitches and winding directions in the layers are adjusted so that the layers have uniform impedance to provide uniform current distribution (see, for example, Japanese Unexamined Patent Application Publication No. 9-45150). As a result of achieving such uniformity in current flow, the AC loss can be reduced to about 1/(the number of layers) as compared with the case in which otherwise nonuniform current flow might occur in the layers.
Another approach has been proposed in which superconducting filaments are twisted in a tape so as to reduce the AC loss of the tape itself, and such superconducting tapes having twisted superconducting filaments are used in a cable in order to obtain a low-loss cable (see, for example, Japanese Unexamined Patent Application Publication No. 7-105753).
However, even when the winding pitches of the tapes are adjusted to achieve the uniformity of current flow across the layers, the AC loss can be reduced to the level of only about 1/(the number of layers) as compared with the AC loss which may be caused in a case of mono-block model.
When superconducting wires or tapes having twisted superconducting filaments (hereinafter such superconducting wires and tapes are both referred to as “twisted filament type superconducting wires”) are used in a cable, the loss can be further reduced. In such case, however, there are problems in that the application of twisting in the process of forming superconducting wires causes a reduction of critical current (Ic) and breakage of filament wires, etc., resulting in the decrease of the yield, the degradation of current-carrying properties of the cable, and the increase of manufacturing cost.